Interaction of ionizing radiation with matter generates, under certain conditions, the emission of electrons from the material surface. Energetic electrons or positrons impinging on a solid tend to lose energy primarily through inelastic collisions with electrons from the material. These electrons, if produced near the surface and possessing sufficient energy to surmount the surface potential barrier of the material, can escape the solid. These electrons are known as secondary electrons. Given the short escape depth usually associated with a secondary electron, secondary electron emission (SEE) is considered a "surface" effect as opposed to a "bulk" mechanism, which generally are used for detection of energetic gamma rays. Because of this, only the characteristics of the last few layers near the surface define the SEE properties for a given material.
The rate at which these phenomena occur can be described by a quantity, (.delta.), known as the secondary-electron emission (SEE) coefficient, i.e., the number of secondary electrons that are ejected for each primary electron incident. Depending on parameters such as incident primary beta energy, the angle of particle incidence, and material composition, the average number of ejected secondary electrons per incident particle can vary from below unity, up to 5 or 6 for some oxides. For some insulators and intermetallic compounds, SEE yields may be as high as 10 to 20.
Existing beta radiation detectors, such as scintillation counters, require the presence of beta particles having sufficient energy to generate a detection event. When a beta particle passes through a suitable material, energy is absorbed, resulting in excitation of electrons in the scintillator. The energy is re-emitted as flashes of light, or scintillations, which are absorbed by the photocathode of a photo-multiplier tube. Photoelectrons are emitted, in turn, and the number of electrons emitted or to be emitted is amplified by the photo-multiplier tube. Typically, a current pulse is produced thereby. The size of the pulse is generally proportional to the number of scintillations produced, and thus to the energy lost by the particle in the scintillator.
Although devices such as scintillators can be used to detect radiation, such as beta-radiation-emitting tracers, in tumors, they have met with limited success due to their sensitivity to background gamma radiation. The range of gamma rays is large in tissue; therefore, a large accumulation of radioactivity in a distant organ can affect probe readings, leading to ambiguity in the location of the tumor. One method for avoiding the problem caused by detection of background gamma radiation is to make the detection device sensitive to only short range radiation.
Because electrons and positrons typically have short ranges in tissue, a probe sensitive only to these charged particles will usually locate radioactivity only when placed in close proximity to the sources of such activity. Although a relatively pure, high-energy beta emitter with no gamma radiation, such as phosphorus-32, can be used as a specific radioactive marker, P-32 is unsuitable for use in many applications, and most other beta-emitting isotopes have gamma emissions as well. Therefore, a detector that is also sensitive to gamma rays is likely to be affected by background gamma radiation.
The high rate of glycolysis in malignant tissue has been successfully exploited as a tumor imaging technique using Positron Emission Tomography (PET) with the glucose analogue radiopharmaceutical 2-.sup.18 F-fluoro-2-deoxy-D-glucose (FDG). Because the original PET scanners were constructed for brain imaging, PET-FDG studies typically concentrated on tumors in the central nervous system. Further developments of whole-body PET scanners extended local tumor localization with FDG to other primary sites. Widespread acceptance during the past decade of FDG as a tumor marker prompted the development of more specific intraoperative beta probes. Although beta particles show great promise for the detection of malignant tissue, the major challenge faced by existing techniques is to construct a device which is highly efficient in detecting beta particles, such as positrons, in the presence of a high annihilation gamma photon background.
One approach to detection of beta radiation, for example, in the context of surgical devices, is described in U.S. Pat. No. 5,008,546 to Mazziotta, et al. (1991), entitled "Intraoperative Beta Probe and Method of Using the Same." Mazziotta described an intraoperative radiation probe that is devised to detect radiolabelled malignant tissues by being selectively sensitive to beta radiation while insensitive to gamma radiation. However, in this device, selectivity is achieved at the price of using two scintillators to detect radiation. One of the scintillators is shielded against beta radiation, while the other is left to detect both beta and gamma radiation. The gamma radiation sensitivity of the dual probes is empirically established and used as a weighted factor to subtract the outputs of the two probes, thereby leaving a signal indicative of the beta radiation emitted by the radiolabelled tissue.
Also, Mazziotta's instrument typically requires calibration for each radio-isotope and the weighted subtraction of both signals is calculated by a computer. Such probes report a sensitivity of 4,000 cps/.mu.Ci for point sources of .sup.18 F using a phantom 5 mm in diameter by 5 mm long with an .sup.18 F concentration of 0.5 .mu.Ci/ml. When this source was placed in a 2.2 liter volume containing a uniform background of .sup.18 F with a concentration of 0.05 .mu.Ci/ml and scanned, a 1 cm FWHM resolution was achieved. However, for some malignancies, metastases may occur well before the primary tumor reaches a size of 1 cm, limiting the clinical usefulness of probes of this type.
Existing beta-sensitive probes typically require extended measurement periods of approximately 10 seconds or longer over a single point to obtain statistically meaningful readings. Furthermore, because they operate by detecting differences in local radiation intensities, it often is necessary to take several measurements over the suspected area. The use of such a device in the operating room could be severely impaired, considering the difficulties of immobilizing areas of tissue for extended periods of time during surgical procedures.
What is needed, therefore, is a highly sensitive beta-radiation probe that is capable of providing information at a faster rate, typically, 1-2 mm per second or higher.